Through the technique of genetic engineering, it has become possible to transfer genes from one organism to another organism. One of the objectives of this technique has been to enable the production of proteins in organisms which do not natively make the proteins. Thus it has become possible, and quite common, to isolate genes for human proteins, to insert those proteins into bacterial hosts, and then to grow up large quantities of the bacteria which contain the human gene and which express the human gene to make the human protein. From the bacterial host, the human protein can then be recovered through appropriate purification processes.
While this technique has proven efficacious and useful for a wide variety of proteins, it does have some limitations. Bacteria are prokaryotic organisms which thus differ in their expression and processing systems for genetic products from eukaryotic organisms, such as mammals. Many human proteins must be glycosylated in order to have proper biological activity. Glycosylation refers to the process by which sugars are added onto polypeptide chains to make glycoproteins. Bacteria are incapable of glycosylating proteins at all, and therefore are not optimal hosts for the production of such glycoproteins. Therefore interest has centered in other eukaryotic protein expression systems, notably yeast and insect cells in culture, as possible hosts for the production of recombinant proteins which might be more appropriately glycosylated.
For this reason, and related reasons, there has been effort directed toward the tissue culturing of insect cells in order to serve as recombinant hosts for the production of human proteins. Several systems have been developed for the culture of insect cells in vitro, and vectors have been developed which are capable of transferring foreign DNA into susceptible insect cells. The transforming vectors are most commonly made from a group of insect pathogenic virus known as Baculoviridae, the viruses being known as Baculoviruses. Baculoviruses are characterized by a circular double-stranded DNA genome and a rod-shaped enveloped virion. Through the use of such Baculovirus vectors, genes can be inserted into insect cells in culture, and human proteins can be made by those insect cells. While the glycosylation pattern of proteins made in insect cells is not identical to that occurring in mammals, limited glycosylation does occur and it may be sufficient, in some instances, to achieve the required degree of biological efficacy.
Another requirement of some proteins for efficacy is lipid-associations. Some mammalian proteins, notably including plasma proteins, require association with lipid particles to form lipoprotein particles in order to have appropriate biological activity. Such proteins have not been efficiently produced in non-mammalian hosts to date.
As stated above, most of the activity in production of proteins in insect cells has been directed toward propagation of insect cells in culture. Such efforts have focused on the fall armyworm, Spodoptera frugiperda. The cell lines have been developed also from other insects such as the cabbage looper, Trichoplusia ni and the silkworm, Bombyx mori. It has also been suggested that analogous cell lines can be created using the tobacco hornworm, or Manduca sexta. It has been proposed that the large scale propagation of cultured insect cells can be used for making recombinant proteins of biological interest, by the culturing of such cells in large scale bioreactors, and then the recovery of the recombinant proteins therefrom.
Another approach to the production of recombinant proteins is based on the use of live insect larvae. Such an approach uses, in effect, the insect larvae as a factory for the manufacture of the desired gene product. The gene must be introduced into the larvae, and allowed to proliferate, and then the hemolymph recovered from the larvae so that the proteins can be isolated therefrom. Baculovirus have been used to introduce genes into silkworm larvae, Bombyx mori. Silkworm was selected because they have been propagated for many years for the production of silk, and therefore the protocols for their cultivation and management are well worked out. Silkworms also have rapid growth, and reasonable disease resistance, so they can be reared in large numbers under sterile conditions. In Japan, in particular, automated feeding machines and artificial diets for silkworms have been developed thereby further facilitating the growth of those insects.